1. Field of the Invention
The present invention relates to the drilling of wells, such as oil and gas wells, and more particularly relates to a method for preventing or minimizing the loss of drilling fluid from a wellbore into porous and permeable formations and for correcting so-called lost circulation of drilling fluids in said well by the use of specific aerated clay-free, non-thixotropic, drilling fluids.
2. Prior Art
In rotary drilling, a bit is attached to the lower end of a hollow drill stem with a fluid circulated down the drill stem, through a passageway in the drill bit and then up an annulus between the drill stem and the wall of the wellbore. Since the drilling fluid is usually circulated in the borehole under a pressure greater than that of the formation, the liquid component of the fluid tends to flow into the permeable formations through the interstices, passages or pores within the formation, however, various fluid loss materials such as lignosulfonate salts, starches and modified starches, or suspended clay particles have been used to reduce this type of fluid loss.
In many cases, however, the amounts of drilling fluid lost from the borehole to the formation depends less on the nature of the fluid loss additives and their manner of reducing such flow, than on the structural characteristics of the strata traversed in the well and on the natural pressure of the fluids in said permeable strata. Thus, in low pressure, highly permeable, porous formations, and especially formations having fissures, cavities or crevices, the fluids easily pass through into the formation, with the result that large volumes of the drilling fluid are lost, causing a loss in circulation of the fluid, which may, in turn, lead to the cessation of drilling, the freezing of the drill string, the collapse of the walls of the borehole, or other undesirable consequences.
The problem of lost circulation is frequently caused or enhances by the weight of the drilling fluid column in the borehole which may exceed the natural formation pressure. This problem has been handled by injecting or incorporating a gasiform fluid, e.g., air or other gas into clay-based, thixotropic drilling fluid.
In the drilling fluid class, clay-based fluids have for years preempted the field, because of the traditional and widely held theory in the field that the viscosity suitable for creating a particle carrying capacity in the drilling fluid could be achieved only with a drilling fluid having thixotropic properties, that is, the viscosity must be supplied by a material that will have sufficient gel strength to prevent the drilled particles from separating from the drilling fluid when agitation of the drilling fluid has ceased, for example, in a holding tank at the surface.
In order to obtain the requisite thixotropy or gel strength, hydratable clay or colloidal clay bodies such as bentonite or fuller's earth have been employed. As a result the drilling fluids are usually referred to as "muds." The use of clay-based drilling muds has provided the means of meeting the two basic requirements of drilling fluids, i.e., cooling and particle removal. However, the clay-based drilling muds have created problems for which solutions are needed. For example, since the clays must be hydrated in order to function, it is not possible to employ hydration inhibitors, such as calcium chloride, or if employed, their pressence must be at a level which will not interfere with the clay hydration. In certain types of shales generally found in the Gulf Coast area of Texas and Louisiana, there is a tendency for the shale to disintegrate by swelling or cracking upon contact with the water if hydration is not limited. Thus, the uninhibited clay-based drilling fluids will be prone to shale disintegration.
The drilled particles and any heaving shale material will be hydrated and taken up by the conventional clay-based drilling fluids. The continued addition of extraneous hydrated solid particles to the drilling fluid will increase the viscosity and necessitated costly and constant thinning and reformulation of the drilling mud to maintain its original properties.
Another serious disadvantage of the clay-based fluids is their susceptibility to the detrimental effect of brines which are often found in drilled formations, particularly Gulf Coast formations. Such brines can have a hydration inhibiting effect, detrimental to the hydration requirement for the clays.
Other disadvantages of clay-based drilled fluids are their (1) tendency to prevent the escape of gas bubbles, when the viscosity of the mud rises too high by the incidental addition of hydratable material, which can result in blow-outs; (2) the need for constant human control and supervision of the clay-based fluids because of the expectable, yet unpredictable, variations in properties; and (3) the formation of a thick cake on the internal surfaces of the wellbore.
These widely used clay-based fluids have been aerated to reduce their density and avoid fluid loss as shown for example, in U.S. Pat. Nos. 2,726,063; 2,818,230 and 3,313,362. However, the disadvantages outlined above are not relieved by aeration and additional problems occur from the aeration. The existance of the gel strength in a drilling fluid make it quite difficult to free the air from the fluid for recycling to the borehole. Unless, a substantial amount of the entrained air is removed from the fluid, the mud pumps cannot pressure up the fluid in their cylinders properly in order to obtain circulation of the whole fluid, i.e., the pumps can not suck enough whole fluid into their intake pipe. Hence, either special treatment of the clay containing fluids is required to remove the entrained gas or over capacity pumps must be employed. Since the aerated drilling fluid in many instances is not needed or employed in the entire drilling operation the clay, thixotropic drilling fluids require extra expense.
The use of gases alone as the drilling fluids avoids the inherent problems of the clay muds, however, the usefulness of gases as drilling fluids is limited. When the formation contains water or hydrocarbons, for example, these liquids seep into the borehole and wet the bore cuttings, which tend to gum and ball on the bit and form mud rings on tools which disrupt the circulation of the gas.
To overcome these problems and those of the muds, some specialized aeration fluids have been proposed. For example, U.S. Pat. No. 3,297,098 discloses the use of foams incorporating a phosphate ester which foams in fresh water and brines; U.S. Pat. No. 3,738,437 discloses a clear fluid such as a brine containing partially hydrolyzed polyarylamides, and U.S. Pat. No. 4,013,568 which discloses water or brine, containing a lignite and a high molecular weight acrylic acid polymer.
It is an advantage of the present invention that a method of drilling with a gas containing fluid is provided which uses a general purpose drilling fluid which is equally suitable for drilling with said gas.